This invention relates to cutting elements such as those used in earth boring bits for drilling earth formations. More specifically, this invention relates to cutting elements incorporating a cutting surface having a cutting edge having a continuous varying radius.
A cutting element 1 (FIG. 1), such as shear cutter mounted on an earth boring bit typically has a cylindrical cemented carbide body 10, i.e. a substrate, having an end face 12 (also referred to herein as an “interface surface”). An ultra hard material layer 18, such as polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or a thermally stable polycrystalline (TSP) material is bonded on the interface surface forming a cutting layer. The cutting layer can have a flat, curved or non-uniform interface surface 12. Cutting elements are mounted in pockets 2 of an earth boring bit, such a drag bit 7, at an angle 8, as shown in FIGS. 1 and 2 and contact the earth formation 11 during drilling along edge 9 over cutting layer 18.
Generally speaking, the process for making a cutting element employs a substrate of cemented tungsten carbide where the tungsten carbide particles are cemented together with cobalt. The carbide body is placed adjacent to a layer of ultra hard material particles such as diamond or cubic boron nitride (CBN) particles along with a binder, such as cobalt, within a refractory metal enclosure (commonly referred to as a “can”), as for example a niobium can, and the combination is subjected to a high temperature at a high pressure where diamond or CBN is thermodynamically stable. This is known as a sintering process. The sintering process results in the re-crystallization and formation of a PCD or PCBN ultra hard material layer on the cemented tungsten carbide substrate, i.e., it results in the formation of a cutting element having a cemented tungsten carbide substrate and an ultra hard material cutting layer. The ultra hard material layer may include tungsten carbide particles and/or small amounts of cobalt. Cobalt promotes the formation of PCD or PCBN. Cobalt may also infiltrate the diamond or CBN from the cemented tungsten carbide substrate.
A TSP is typically formed by “leaching” the cobalt from the diamond lattice structure of PCD. When formed, PCD comprises individual diamond crystals that are interconnected defining a lattice structure. Cobalt particles are often found within the interstitial spaces in the diamond lattice structure. Cobalt has a significantly different coefficient of thermal expansion as compared to diamond, and as such upon heating of the PCD, the cobalt expands, causing cracking to form in the lattice structure, resulting in the deterioration of the PCD layer. By removing, i.e., by leaching, the cobalt from the diamond lattice structure, the PCD layer becomes more heat resistant, i.e., more thermally stable. However, the polycrystalline diamond layer becomes more brittle. Accordingly, in certain cases, only a select portion, measured either in depth or width, of the PCD layer is leached in order to gain thermal stability without losing impact resistance. A TSP material may also be formed by forming PCD with a thermally compatible silicon carbide binder instead of cobalt.
The cemented tungsten carbide substrate is typically formed by placing tungsten carbide powder and a binder in a mold and then heating the binder to melting temperature causing the binder to melt and infiltrate the tungsten carbide particles fusing them together and cementing the substrate. Alternatively, the tungsten carbide powder may be cemented by the binder during the high temperature, high pressure sintering process used to re-crystallize the ultra hard material layer. In such case, the substrate material powder along with the binder are placed in the refractory metal enclosure. Ultra hard material particles are provided over the substrate material to form the ultra hard material polycrystalline layer. The entire assembly is then subjected to a high temperature, high pressure process forming the cutting element having a substrate and a polycrystalline ultra hard material layer over it.
In many instances the cutting edge of the cutting layer, which contacts the earth formation during drilling, such as edge 9, has sharp edges. These sharp edges may be defined by the intersection of the upper and circumferential surfaces defining the cutting layer or by chamfers formed on the cutting edge. These sharp edges create stress concentrations which may cause cracking and chipping of the cutting layer.